Organic/inorganic composite porous film and electrochemical device prepared thereby

ABSTRACT

Disclosed is an organic/inorganic composite porous film comprising: (a) a porous substrate having pores; and (b) an active layer formed by coating a surface of the substrate or a part of the pores in the substrate with a mixture of inorganic particles and a binder polymer, wherein the inorganic particles in the active layer are interconnected among themselves and are fixed by the binder polymer, and interstitial volumes among the inorganic particles form a pore structure. A method for manufacturing the same film and an electrochemical device including the same film are also disclosed. An electrochemical device comprising the organic/inorganic composite porous film shows improved safety and quality, simultaneously.

TECHNICAL FIELD

The present invention relates to a novel organic/inorganic compositeporous film that can show excellent thermal safety and lithium ionconductivity and a high degree of swelling with electrolyte compared toconventional polyolefin-based separators, and an electrochemical devicecomprising the same, which ensures safety and has improved quality.

BACKGROUND ART

Recently, there is an increasing interest in energy storage technology.Batteries have been widely used as energy sources in portable phones,camcorders, notebook computers, PCs and electric cars, resulting inintensive research and development into them. In this regard,electrochemical devices are subjects of great interest. Particularly,development of rechargeable secondary batteries is the focus ofattention.

Secondary batteries are chemical batteries capable of repeated chargeand discharge cycles by means of reversible interconversion betweenchemical energy and electric energy, and may be classified into Ni-MHsecondary batteries and lithium secondary batteries. Lithium secondarybatteries include secondary lithium metal batteries, secondary lithiumion batteries, secondary lithium polymer batteries, secondary lithiumion polymer batteries, etc.

Because lithium secondary batteries have drive voltage and energydensity higher than those of conventional batteries using aqueouselectrolytes (such as Ni-MH batteries), they are produced commerciallyby many production companies. However, most lithium secondary batterieshave different safety characteristics depending on several factors.Evaluation of and security in safety of batteries are very importantmatters to be considered. Therefore, safety of batteries is strictlyrestricted in terms of ignition and combustion in batteries by safetystandards.

Currently available lithium ion batteries and lithium ion polymerbatteries use polyolefin-based separators in order to prevent shortcircuit between a cathode and an anode. However, because suchpolyolefin-based separators have a melting point of 200° C. or less,they have a disadvantage in that they can be shrunk or molten to cause achange in volume when the temperature of a battery is increased byinternal and/or external factors. Therefore, there is a greatpossibility of short-circuit between a cathode and an anode caused byshrinking or melting of separators, resulting in accidents such asexplosion of a battery caused by emission of electric energy. As aresult, it is necessary to provide a separator that does not cause heatshrinking at high temperature.

To solve the above problems related with polyolefin-based separators,many attempts are made to develop an electrolyte using an inorganicmaterial serving as a substitute for a conventional separator. Suchelectrolytes may be broadly classified into two types. The first type isa solid composite electrolyte obtained by using inorganic particleshaving lithium ion conductivity alone or by using inorganic particleshaving lithium ion conductivity mixed with a polymer matrix. See,Japanese Laid-Open Patent No. 2003-022707, [“Solid State Ionics”-vol.158, n. 3, p. 275, (2003)], [“Journal of Power Sources”-vol. 112, n. 1,p. 209, (2002)], [“Electrochimica Acta”-vol. 48, n. 14, p. 2003,(2003)], etc. However, it is known that such composite electrolytes arenot advisable, because they have low ion conductivity compared to liquidelectrolytes and the interfacial resistance between the inorganicmaterials and the polymer is high while they are mixed.

The second type is an electrolyte obtained by mixing inorganic particleshaving lithium ion conductivity or not with a gel polymer electrolyteformed of a polymer and liquid electrolyte. In this case, inorganicmaterials are introduced in a relatively small amount compared to thepolymer and liquid electrolyte, and thus merely have a supplementaryfunction to assist in lithium ion conduction made by the liquidelectrolyte.

As described above, electrolytes according to the prior art usinginorganic particles have common problems as follows. First, when liquidelectrolyte is not used, the interfacial resistance among inorganicparticles and between inorganic particles and polymer excessivelyincreases, resulting in degradation of quality. Next, theabove-described electrolytes cannot be easily handled due to thebrittleness thereof when an excessive amount of inorganic materials isintroduced. Therefore, it is difficult to assemble batteries using suchelectrolytes. Particularly, most attempts made up to date are fordeveloping an inorganic material-containing composite electrolyte in theform of a free standing film. However, it is practically difficult toapply such electrolyte in batteries due to poor mechanical propertiessuch as high brittleness of the film. Even if the content of inorganicparticles is reduced to improve mechanical properties, mixing inorganicparticles with a liquid electrolyte causes a significant drop inmechanical properties due to the liquid electrolyte, resulting in a failin the subsequent assemblage step of batteries. When a liquidelectrolyte is injected after assemblage of a battery, dispersion of theelectrolyte in a battery needs too long time and actual wettability withelectrolyte is poor due to the high content of the polymer in theorganic/inorganic composite film. Additionally, addition of inorganicparticles for improving safety causes a problem of a significant drop inlithium ion conductivity. Further, because the electrolyte has no porestherein or, if any, has pores with a size of several angstroms (Å) andlow porosity, the electrolyte cannot sufficiently serve as separator.

In addition, U.S. Pat. No. 6,432,586 discloses a composite filmcomprising a polyolefin-based separator coated with silica, etc., so asto improve the mechanical properties such as brittleness of theorganic/inorganic composite electrolyte. However, because such filmsstill use a polyolefin-based separator, they have a disadvantage in thatit is not possible to obtain a significant improvement in safetyincluding prevention of heat shrinking at high temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic view showing an organic/inorganic composite porousfilm according to the present invention;

FIG. 2 is a photograph taken by a Scanning Electron Microscope (SEM)showing the organic/inorganic composite porous film (PVdF-CTFE/BaTiO₃)according to Example 1;

FIG. 3 is a photograph taken by SEM showing a polyolefin-based separator(PP/PE/PP) used in Comparative Example 1;

FIG. 4 is a photograph taken by SEM showing a conventional film(Al₂O₃—SiO₂/PET nonwoven) using no binder polymer according to the priorart;

FIG. 5 is a photograph showing the organic/inorganic composite porousfilm (PVdF-CTFE/BaTiO₃) according to Example 1 compared to a currentlyused PP/PE/PP separator and the organic/inorganic composite porous film(PVdF-HFP/BaTiO₃) according to Comparative Example 3, which has aninorganic material layer formed on a PP/PE/PP separator, after each ofthe samples are maintained at 150° C. for 1 hour;

FIG. 6 is a picture and graph showing the results of an overcharge testfor the lithium secondary battery including a currently used PP/PE/PPseparator according to Comparative Example 1 and the lithium secondarybattery including the organic/inorganic composite porous film(PVdF-CTFE/BaTiO₃) according to Example 1;

FIG. 7 is a graph showing the high rate discharge characteristics(C-rate) of the lithium secondary battery including a currently usedPP/PE/PP separator according to Comparative Example 1 and the lithiumsecondary battery including the organic/inorganic composite porous film(PVdF-CTFE/BaTiO₃) according to Example 1; and

FIG. 8 is a graph showing the cycle characteristics of the lithiumsecondary battery including a currently used PP/PE/PP separatoraccording to Comparative Example 1 and the lithium secondary batteryincluding the organic/inorganic composite porous film (PVdF-CTFE/BaTiO₃)according to Example 1.

DISCLOSURE OF THE INVENTION

We have found that an organic/inorganic composite porous film, formed byusing (1) a heat resistant porous substrate, (2) inorganic particles and(3) a binder polymer, improves poor thermal safety of a conventionalpolyolefin-based separator. Additionally, we have found that because theorganic/inorganic composite porous film has pore structures present bothin the porous substrate and in an active layer formed of the inorganicparticles and the binder polymer coated on the porous substrate, itprovides an increased volume of space, into which a liquid electrolyteinfiltrates, resulting in improvements in lithium ion conductivity anddegree of swelling with electrolyte. Therefore, the organic/inorganiccomposite porous film can improve the quality and safety of anelectrochemical device using the same as separator.

Therefore, it is an object of the present invention to provide anorganic/inorganic composite porous film capable of improving the qualityand safety of an electrochemical device, a method for manufacturing thesame and an electrochemical device comprising the same.

According to an aspect of the present invention, there is provided anorganic/inorganic composite porous film, which comprises (a) a poroussubstrate having pores; and (b) an active layer formed by coating asurface of the substrate or a part of the pores in the substrate with amixture of inorganic particles and a binder polymer, wherein theinorganic particles in the active layer are interconnected amongthemselves and are fixed by the binder polymer, and interstitial volumesamong the inorganic particles form a pore structure. There is alsoprovided an electrochemical device (preferably, a lithium secondarybattery) comprising the same.

According to another aspect of the present invention, there is provideda method for manufacturing an organic/inorganic composite porous film,which includes the steps of: (a) dissolving a binder polymer into asolvent to form a polymer solution; (b) adding inorganic particles tothe polymer solution obtained from step (a) and mixing them; and (c)coating the mixture of inorganic particles with a binder polymerobtained from step (b) on the surface of a substrate having pores or ona part of the pores in the substrate, followed by drying.

Hereinafter, the present invention will be explained in more detail.

The present invention is characterized in that it provides a novelorganic/inorganic composite porous film, which serves sufficiently asseparator to prevent electrical contact between a cathode and an anodeof a battery and to pass ions therethrough, improves poor thermal safetyrelated with a conventional polyolefin-based separator, and showsexcellent lithium ion conductivity and a high degree of swelling withelectrolyte.

The organic/inorganic composite porous film is obtained by coating amixture of inorganic particles with a binder polymer on the surface of aporous substrate (preferably, a heat resistant substrate having amelting point of 200° C. or higher). The pores present in the substrateitself and a uniform pore structure formed in the active layer by theinterstitial volumes among the inorganic particles permit theorganic/inorganic composite porous film to be used as separator.Additionally, if a polymer capable of being gelled when swelled with aliquid electrolyte is used as the binder polymer component, theorganic/inorganic composite porous film can serve also as electrolyte.

Particular characteristics of the organic/inorganic composite porousfilm are as follows.

(1) Conventional solid electrolytes formed by using inorganic particlesand a binder polymer have no pore structure or, if any, have anirregular pore structure having a pore size of several angstroms.Therefore, they cannot serve sufficiently as spacer, through whichlithium ions can pass, resulting in degradation in the quality of abattery. On the contrary, the organic/inorganic composite porous filmaccording to the present invention has uniform pore structures both inthe porous substrate and in the active layer as shown in FIGS. 1 and 2,and the pore structures permit lithium ions to move smoothlytherethrough. Therefore, it is possible to introduce a large amount ofelectrolyte through the pore structures so that a high degree ofswelling with electrolyte can be obtained, resulting in improvement ofbattery quality.

(2) Conventional separators or polymer electrolytes are formed in theshape of free standing films and then assembled together withelectrodes. On the contrary, the organic/inorganic composite porous filmaccording to the present invention is formed by coating it directly onthe surface of a porous substrate having pores so that the pores on theporous substrate and the active layer can be anchored to each other,thereby providing a firm physical bonding between the active layer andthe porous substrate. Therefore, problems related with mechanicalproperties such as brittleness can be improved. Additionally, suchincreased interfacial adhesion between the porous substrate and theactive coating layer can decrease the interfacial resistance. In fact,the organic/inorganic composite porous film according to the presentinvention includes the organic/inorganic composite active layer bondedorganically to the porous substrate. Additionally, the active layer doesnot affect the pore structure present in the porous substrate so thatthe structure can be maintained. Further, the active layer itself has auniform pore structure formed by the inorganic particles (see FIGS. 1and 2). Because the above-mentioned pore structures are filled with aliquid electrolyte injected subsequently, interfacial resistancegenerated among the inorganic particles or between the inorganicparticles and the binder polymer can be decreased significantly.

(3) The organic/inorganic composite porous film according to the presentinvention shows improved thermal safety by virtue of the heat resistantsubstrate and inorganic particles.

In other words, although conventional polyolefin-based separators causeheat shrinking at high temperature because they have a melting point of120-140° C., the organic/inorganic composite porous film does not causeheat shrinking due to the heat resistance of the porous substrate havinga melting point of 200° C. or higher and the inorganic particles.Therefore, an electrochemical device using the above organic/inorganiccomposite porous film as separator causes no degradation in safetyresulting from an internal short circuit between a cathode and an anodeeven under extreme conditions such as high temperature, overcharge, etc.As a result, such electrochemical devices have excellent safetycharacteristics compared to conventional batteries.

(4) Nonwoven webs made of PET having a mixed layer of alumina (Al₂O₃)and silica (SiO₂) are known to one skilled in the art. However, suchcomposite films use no binder polymer for supporting and interconnectinginorganic particles. Additionally, there is no correct understandingwith regard to the particle diameter and homogeneity of the inorganicparticles and a pore structure formed by the inorganic particles.Therefore, such composite films according to the prior art have aproblem in that they cause degradation in the quality of a battery (see,FIG. 4). More particularly, when the inorganic particles have arelatively large diameter, the thickness of an organic/inorganic coatinglayer obtained under the same solid content increases, resulting indegradation in mechanical properties. Additionally, in this case, thereis a great possibility of internal short circuit during charge/dischargecycles of a battery due to an excessively large pore size. Further, dueto the lack of a binder that serves to fix the inorganic particles onthe substrate, a finally formed film is deteriorated in terms ofmechanical properties and has a difficult in applying to a practicalbattery assemblage process. For example, the composite films accordingto the prior art may not be amenable to a lamination process. On thecontrary, we have recognized that control of the porosity and pore sizeof the organic/inorganic composite porous film according to the presentinvention is one of the factors affecting the quality of a battery.Therefore, we have varied and optimized the particle diameter of theinorganic particles or the mixing ratio of the inorganic particles withthe binder polymer. In addition, according to the present invention, thebinder polymer used in the active layer can serve, as binder, tointerconnect and stably fix the inorganic particles among themselves,between the inorganic particles and the surface of the heat resistantporous substrate, and between the inorganic particles and a part of thepores in the substrate, thereby preventing degradation in mechanicalproperties of a finally formed organic/inorganic composite porous film.

(5) When the inorganic particles used in the active layer of theorganic/inorganic composite porous film have high dielectric constantand/or lithium ion conductivity, the inorganic particles can improvelithium ion conductivity as well as heat resistance, therebycontributing to improvement of battery quality.

(6) When the binder polymer used in the organic/inorganic compositeporous film is one showing a high degree of swelling with electrolyte,the electrolyte injected after assemblage of a battery can infiltrateinto the polymer and the resultant polymer containing the electrolyteinfiltrated therein has a capability of conducting electrolyte ions.Therefore, the organic/inorganic composite porous film according to thepresent invention can improve the quality of an electrochemical devicecompared to conventional organic/inorganic composite electrolytes.Additionally, the organic/inorganic composite porous film providesadvantages in that wettablity with an electrolyte for battery isimproved compared to conventional hydrophobic polyolefin-basedseparators, and use of a polar electrolyte for battery can be permitted.

(7) Finally, if the binder polymer is one capable of being gelled whenswelled with electrolyte, the polymer reacts with the electrolyteinjected subsequently and is gelled, thereby forming a gel typeorganic/inorganic composite electrolyte. Such electrolytes are producedwith ease compared to conventional gel-type electrolytes and showexcellent ion conductivity and a high degree of swelling withelectrolyte, thereby contributing to improve the quality of a battery.

In the organic/inorganic composite porous film according to the presentinvention, there is no particular limitation in the substrate coatedwith the mixture of inorganic particles and binder polymer, as long asit is a porous substrate having pores. However, it is preferable to usea heat resistant porous substrate having a melting point of 200° C. orhigher. Such heat resistant porous substrates can improve the thermalsafety of the organic/inorganic composite porous film under externaland/or internal thermal impacts.

Non-limiting examples of the porous substrate that may be used includepolyethylene terephthalate, polybutylene terephthalate, polyester,polyacetal, polyamide, polycarbonate, polyimide, polyetherether ketone,polyether sulfone, polyphenylene oxide, polyphenylene sulfidro,polyethylene naphthalene or mixtures thereof. However, other heatresistant engineering plastics may be used with no particularlimitation.

Although there is no particular limitation in thickness of the poroussubstrate, the porous substrate preferably has a thickness of between 1μm and 100 μm, more preferably of between 5 μm and 50 μm. When theporous substrate has a thickness of less than 1 μm, it is difficult tomaintain mechanical properties. When the porous substrate has athickness of greater than 100 μm, it may function as resistance layer.

Although there is no particular limitation in pore size and porosity ofthe porous substrate, the porous substrate preferably has a porosity ofbetween 5% and 95%. The pore size (diameter) preferably ranges from 0.01μm to 50 μm, more preferably from 0.1 μm to 20 μm. When the pore sizeand porosity are less than 0.01 μm and 5%, respectively, the poroussubstrate may function as resistance layer. When the pore size andporosity are greater than 50 μm and 95%, respectively, it is difficultto maintain mechanical properties.

The porous substrate may take the form of a membrane or fiber. When theporous substrate is fibrous, it may be a nonwoven web forming a porousweb (preferably, spunbond type web comprising long fibers or melt blowntype web).

A spunbond process is performed continuously through a series of stepsand provides long fibers formed by heating and melting, which isstretched, in turn, by hot air to form a web. A melt blown processperforms spinning of a polymer capable of forming fibers through aspinneret having several hundreds of small orifices, and thus providesthree-dimensional fibers having a spider-web structure resulting frominterconnection of microfibers having a diameter of 10 μm or less.

In the organic/inorganic composite porous film according to the presentinvention, one component present in the active layer formed on thesurface of the porous substrate or on a part of the pores in the poroussubstrate is inorganic particles currently used in the art. Theinorganic particles permit an interstitial volume to be formed amongthem, thereby serving to form micropores and to maintain the physicalshape as spacer. Additionally, because the inorganic particles arecharacterized in that their physical properties are not changed even ata high temperature of 200° C. or higher, the organic/inorganic compositeporous film using the inorganic particles can have excellent heatresistance.

There is no particular limitation in selection of inorganic particles,as long as they are electrochemically stable. In other words, there isno particular limitation in inorganic particles that may be used in thepresent invention, as long as they are not subjected to oxidation and/orreduction at the range of drive voltages (for example, 0-5 V based onLi/Li⁺) of a battery, to which they are applied. Particularly, it ispreferable to use inorganic particles having ion conductivity as high aspossible, because such inorganic particles can improve ion conductivityand quality in an electrochemical device. Additionally, when inorganicparticles having a high density are used, they have a difficulty indispersion during a coating step and may increase the weight of abattery to be manufactured. Therefore, it is preferable to use inorganicparticles having a density as low as possible. Further, when inorganicparticles having a high dielectric constant are used, they cancontribute to increase the dissociation degree of an electrolyte salt ina liquid electrolyte, such as a lithium salt, thereby improving the ionconductivity of the electrolyte.

For these reasons, it is preferable to use inorganic particles having ahigh dielectric constant of 5 or more, preferably of 10 or more,inorganic particles having lithium conductivity or mixtures thereof.

Particular non-limiting examples of inorganic particles having adielectric constant of 5 or more include BaTiO₃, Pb(Zr,Ti)O₃ (PZT),Pb_(1-x)La_(x)Zr_(1-y)Ti_(y)O₃ (PLZT), PB(Mg₃Nb_(2/3))O₃—PbTiO₃(PMN-PT), hafnia (HfO₂), SrTiO₃, SnO₂, CeO₂, MgO, NiO, CaO, ZnO, ZrO₂,Y₂O₃, Al₂O₃, TiO₂ or mixtures thereof.

As used herein, “inorganic particles having lithium ion conductivity”are referred to as inorganic particles containing lithium elements andhaving a capability of conducting lithium ions without storing lithium.Inorganic particles having lithium ion conductivity can conduct and movelithium ions due to defects present in their structure, and thus canimprove lithium ion conductivity and contribute to improve batteryquality. Non-limiting examples of such inorganic particles havinglithium ion conductivity include: lithim phosphate (Li₃PO₄), lithiumtitanium phosphate (Li_(x)Ti_(y)(PO₄)₃, 0<x<2, 0<y<3), lithium aluminiumtitanium phosphate (Li_(x)Al_(y)Ti_(z)(PO₄)₃, 0<x<2, 0<y<1, 0<z<3),(LiAlTiP)_(x)O_(y) type glass (0<x<4, 0<y<13) such as14Li₂O-9Al₂O₃-38TiO₂-39P₂O₅, lithium lanthanum titanate(Li_(x)La_(y)TiO₃, 0<x<2, 0<y<3), lithium germanium thiophosphate(Li_(x)Ge_(y)P_(z)S_(w), 0<x<4, 0<y<1, 0<z<1, 0<w<5), such asLi_(3.25)Ge_(0.25)P_(0.75)S₄, lithium nitrides (Li_(x)N_(y), 0<x<4,0<y<2) such as Li₃N, SiS₂ type glass (Li_(x)Si_(y)S_(z), 0<x<3, 0<y<2,0<z<4) such as Li₃PO₄—Li₂S—SiS₂, P₂S₅ type glass (Li_(x)P_(y)S_(z),0<x<3, 0<y<3, 0<z<7) such as LiI—Li₂S—P₂S₅, or mixtures thereof.

According to the present invention, inorganic particles havingrelatively high dielectric constant are used instead of inorganicparticles having no reactivity or having a relatively low dielectricconstant. Further, the present invention also provides a novel use ofinorganic particles as separators.

The above-described inorganic particles, that have never been used asseparators, for example Pb(Zr,Ti)O₃ (PZT),Pb_(1-x)La_(x)Zr_(1-y)Ti_(y)O₃ (PLZT), Pb(Mg₃Nb_(2/3))O₃—PbTiO₃(PMN-PT), hafnia (HfO₂), etc., have a high dielectric constant of 100 ormore. The inorganic particles also have piezoelectricity so that anelectric potential can be generated between both surfaces by the chargeformation, when they are drawn or compressed under the application of acertain pressure. Therefore, the inorganic particles can preventinternal short circuit between both electrodes, thereby contributing toimprove the safety of a battery. Additionally, when such inorganicparticles having a high dielectric constant are combined with inorganicparticles having lithium ion conductivity, synergic effects can beobtained.

The organic/inorganic composite porous film according to the presentinvention can form pores having a size of several micrometers bycontrolling the size of inorganic particles, content of inorganicparticles and the mixing ratio of inorganic particles and binderpolymer. It is also possible to control the pore size and porosity.

Although there is no particular limitation in size of inorganicparticles, inorganic particles preferably have a size of 0.001-10 μm forthe purpose of forming a film having a uniform thickness and providing asuitable porosity. When the size is less than 0.001 μm, inorganicparticles have poor dispersibility so that physical properties of theorganic/inorganic composite porous film cannot be controlled with ease.When the size is greater than 10 μm, the resultant organic/inorganiccomposite porous film has an increased thickness under the same solidcontent, resulting in degradation in mechanical properties. Furthermore,such excessively large pores may increase a possibility of internalshort circuit being generated during repeated charge/discharge cycles.

The inorganic particles are present in the mixture of the inorganicparticles with binder polymer forming the organic/inorganic compositeporous film, preferably in an amount of 50-99 wt %, more particularly inan amount of 60-95 wt % based on 100 wt % of the total weight of themixture. When the content of the inorganic particles is less than 50 wt%, the binder polymer is present in such a large amount as to decreasethe interstitial volume formed among the inorganic particles and thus todecrease the pore size and porosity, resulting in degradation in thequality of a battery. When the content of the inorganic particles isgreater than 99 wt %, the polymer content is too low to providesufficient adhesion among the inorganic particles, resulting indegradation in mechanical properties of a finally formedorganic/inorganic composite porous film.

In the organic/inorganic composite porous film according to the presentinvention, another component present in the active layer formed on thesurface of the porous substrate or on a part of the pores in the poroussubstrate is a binder polymer currently used in the art. The binderpolymer preferably has a glass transition temperature (T_(g)) as low aspossible, more preferably T_(g) of between −200° C. and 200° C. Binderpolymers having a low T_(g) as described above are preferable, becausethey can improve mechanical properties such as flexibility andelasticity of a finally formed film. The polymer serves as binder thatinterconnects and stably fixes the inorganic particles among themselves,and thus prevents degradation in mechanical properties of a finallyformed organic/inorganic composite porous film.

When the binder polymer has ion conductivity, it can further improve thequality of an electrochemical device. However, it is not essential touse a binder polymer having ion conductivity. Therefore, the binderpolymer preferably has a dielectric constant as high as possible.Because the dissociation degree of a salt in an electrolyte depends onthe dielectric constant of a solvent used in the electrolyte, thepolymer having a higher dielectric constant can increase thedissociation degree of a salt in the electrolyte used in the presentinvention. The dielectric constant of the binder polymer may range from1.0 to 100 (as measured at a frequency of 1 kHz), and is preferably 10or more.

In addition to the above-described functions, the binder polymer used inthe present invention may be further characterized in that it is gelledwhen swelled with a liquid electrolyte, and thus shows a high degree ofswelling. Therefore, it is preferable to use a polymer having asolubility parameter of between 15 and 45 MPa^(1/2), more preferably ofbetween 15 and 25 MPa^(1/2), and between 30 and 45 MPa^(1/2). Therefore,hydrophilic polymers having a lot of polar groups are more preferablethan hydrophobic polymers such as polyolefins. When the binder polymerhas a solubility parameter of less than 15 Mpa^(1/2) or greater than 45Mpa^(1/2), it has a difficulty in swelling with a conventional liquidelectrolyte for battery.

Non-limiting examples of the binder polymer that may be used in thepresent invention include polyvinylidenefluoride-co-hexafluoropropylene, polyvinylidenefluoride-co-trichloroethylene, polymethylmethacrylate,polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate,polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate,cellulose acetate butyrate, cellulose acetate propionate,cyanoethylpullulan, cyanoethyl polyvinylalcohol, cyanoethylcellulose,cyanoethylsucrose, pullulan, carboxymetyl cellulose,acrylonitrile-styrene-butadiene copolymer, polyimide or mixturesthereof. Other materials may be used alone or in combination, as long asthey satisfy the above characteristics.

The organic/inorganic composite porous film may further compriseadditives other than the inorganic particles and binder polymer as stillanother component of the active layer.

As described above, the organic/inorganic composite porous film formedby coating the mixture of inorganic particles with binder polymer ontothe porous substrate has pores contained in the porous substrate itselfand forms pore structures in the substrate as well as in the activelayer due to the interstitial volume among the inorganic particles,formed on the substrate. The pore size and porosity of theorganic/inorganic composite porous film mainly depend on the size ofinorganic particles. For example, when inorganic particles having aparticle diameter of 1 μm or less are used, pores formed thereby alsohave a size of 1 μm or less. The pore structure is filled with anelectrolyte injected subsequently and the electrolyte serves to conductions. Therefore, the size and porosity of the pores are importantfactors in controlling the ion conductivity of the organic/inorganiccomposite porous film. Preferably, the pores size and porosity of theorganic/inorganic composite porous film according to the presentinvention range from 0.01 to 10 μm and from 5 to 95%, respectively.

There is no particular limitation in thickness of the organic/inorganiccomposite porous film according to the present invention. The thicknessmay be controlled depending on the quality of a battery. According tothe present invention, the film preferably has a thickness of between 1and 100 μm, more preferably of between 2 and 30 μm. Control of thethickness of the film may contribute to improve the quality of abattery.

There is no particular limitation in mixing ratio of inorganic particlesto binder polymer in the organic/inorganic composite porous filmaccording to the present invention. The mixing ratio can be controlledaccording to the thickness and structure of a film to be formed finally.

The organic/inorganic composite porous film may be applied to a batterytogether with a microporous separator (for example, a polyolefin-basedseparator), depending on the characteristics of a finally formedbattery.

The organic/inorganic composite porous film may be manufactured by aconventional process known to one skilled in the art. One embodiment ofa method for manufacturing the organic/inorganic composite porous filmaccording to the present invention, includes the steps of: (a)dissolving a binder polymer into a solvent to form a polymer solution;(b) adding inorganic particles to the polymer solution obtained fromstep (a) and mixing them; and (c) coating the mixture of inorganicparticles with binder polymer obtained from step (b) on the surface of asubstrate having pores or on a part of the pores in the substrate,followed by drying.

Hereinafter, the method for manufacturing the organic/inorganiccomposite porous film according to the present invention will beexplained in detail.

(1) First, a binder polymer is dissolved in a suitable organic solventto provide a polymer solution.

It is preferable that the solvent has a solubility parameter similar tothat of the polymer to be used and a low boiling point. Such solventscan be mixed uniformly with the polymer and can be removed easily aftercoating the polymer. Non-limiting examples of the solvent that may beused include acetone, tetrahydrofuran, methylene chloride, chloroform,dimethylformamide, N-methyl-2-pyrrolidone, cyclohexane, water ormixtures thereof.

(2) Next, inorganic particles are added to and dispersed in the polymersolution obtained from the preceding step to provide a mixture ofinorganic particles with binder polymer.

It is preferable to perform a step of pulverizing inorganic particlesafter adding the inorganic particles to the binder polymer solution. Thetime needed for pulverization is suitably 1-20 hours. The particle sizeof the pulverized particles ranges preferably from 0.001 and 10 μm.Conventional pulverization methods, preferably a method using a ballmill, may be used.

Although there is no particular limitation in composition of the mixturecontaining inorganic particles and binder polymer, such composition cancontribute to control the thickness, pore size and porosity of theorganic/inorganic composite porous film to be formed finally.

In other words, as the weight ratio (I/P) of the inorganic particles (I)to the polymer (P) increases, porosity of the organic/inorganiccomposite porous film according to the present invention increases.Therefore, the thickness of the organic/inorganic composite porous filmincreases under the same solid content (weight of the inorganicparticles+weight of the binder polymer). Additionally, the pore sizeincreases in proportion to the pore formation among the inorganicparticles. When the size (particle diameter) of the inorganic particlesincreases, interstitial distance among the inorganic particlesincreases, thereby increasing the pore size.

(3) The mixture of inorganic particles with binder polymer is coated onthe heat resistant porous substrate, followed by drying to provide theorganic/inorganic composite porous film.

In order to coat the porous substrate with the mixture of inorganicparticles and binder polymer, any methods known to one skilled in theart may be used. It is possible to use various processes including dipcoating, die coating, roll coating, comma coating or combinationsthereof. Additionally, when the mixture containing inorganic particlesand polymer is coated on the porous substrate, either or both surfacesof the porous substrate may be coated.

The organic/inorganic composite porous film according to the presentinvention, obtained as described above, may be used as separator in anelectrochemical device, preferably in a lithium secondary battery. Ifthe binder polymer used in the film is a polymer capable of being gelledwhen swelled with a liquid electrolyte, the polymer may react with theelectrolyte injected after assembling a battery by using the separator,and thus be gelled to form a gel type organic/inorganic compositeelectrolyte.

The gel type organic/inorganic composite electrolyte according to thepresent invention is prepared with ease compared to gel type polymerelectrolytes according to the prior art, and has a large space to befilled with a liquid electrolyte due to its microporous structure,thereby showing excellent ion conductivity and a high degree of swellingwith electrolyte, resulting in improvement in the quality of a battery.

Further, the present invention provides an electrochemical devicecomprising: (a) a cathode; (b) an anode; (c) the organic/inorganiccomposite porous film according to the present invention, interposedbetween the cathode and anode; and (d) an electrolyte.

Such electrochemical devices include any devices in whichelectrochemical reactions occur and particular examples thereof includeall kinds of primary batteries, secondary batteries, fuel cells, solarcells or capacitors. Particularly, the electrochemical device is alithium secondary battery including a lithium metal secondary battery,lithium ion secondary battery, lithium polymer secondary battery orlithium ion polymer secondary battery.

According to the present invention, the organic/inorganic compositeporous film contained in the electrochemical device serves as separator.If the polymer used in the film is a polymer capable of being gelledwhen swelled with electrolyte, the film may serve also as electrolyte.In addition to the above organic/inorganic composite porous film, amicroporous separator, for example a polyolefin-based separator may beused together.

The electrochemical device may be manufactured by a conventional methodknown to one skilled in the art. In one embodiment of the method formanufacturing the electrochemical device, the electrochemical device isassembled by using the organic/inorganic composite porous filminterposed between a cathode and anode, and then an electrolyte isinjected.

The electrode that may be applied together with the organic/inorganiccomposite porous film according to the present invention may be formedby applying an electrode active material on a current collectoraccording to a method known to one skilled in the art. Particularly,cathode active materials may include any conventional cathode activematerials currently used in a cathode of a conventional electrochemicaldevice. Particular non-limiting examples of the cathode active materialinclude lithium intercalation materials such as lithium manganeseoxides, lithium cobalt oxides, lithium nickel oxides, lithium ironoxides or composite oxides thereof. Additionally, anode active materialsmay include any conventional anode active materials currently used in ananode of a conventional electrochemical device. Particular non-limitingexamples of the anode active material include lithium intercalationmaterials such as lithium metal, lithium alloys, carbon, petroleum coke,activated carbon, graphite or other carbonaceous materials. Non-limitingexamples of a cathode current collector include foil formed of aluminum,nickel or a combination thereof. Non-limiting examples of an anodecurrent collector include foil formed of copper, gold, nickel, copperalloys or a combination thereof.

The electrolyte that may be used in the present invention includes asalt represented by the formula of A⁺B⁻, wherein A⁺ represents an alkalimetal cation selected from the group consisting of Li⁺, Na⁺, K⁺ andcombinations thereof, and B⁻ represents an anion selected from the groupconsisting of PF₆ ⁻, BF₄ ⁻, Cl⁻, Br⁻, I⁻, ClO₄ ⁻, AsF₆ ⁻, CH₃CO₂ ⁻,CF₃SO₃ ⁻, N(CF₃SO₂)₂ ⁻, C(CF₂SO₂)₃ ⁻ and combinations thereof, the saltbeing dissolved or dissociated in an organic solvent selected from thegroup consisting of propylene carbonate (PC), ethylene carbonate (EC),diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate(DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane,diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP),ethylmethyl carbonate (EMC), gamma-butyrolactone (GBL) and mixturesthereof. However, the electrolyte that may be used in the presentinvention is not limited to the above examples.

More particularly, the electrolyte may be injected in a suitable stepduring the manufacturing process of an electrochemical device, accordingto the manufacturing process and desired properties of a final product.In other words, electrolyte may be injected, before an electrochemicaldevice is assembled or in a final step during the assemblage of anelectrochemical device.

Processes that may be used for applying the organic/inorganic compositeporous film to a battery include not only a conventional winding processbut also a lamination (stacking) and folding process of a separator andelectrode.

When the organic/inorganic composite porous film according to thepresent invention is applied to a lamination process, it is possible tosignificantly improve the thermal safety of a battery, because a batteryformed by a lamination and folding process generally shows more severeheat shrinking of a separator compared to a battery formed by a windingprocess. Additionally, when a lamination process is used, there is anadvantage in that a battery can be assembled with ease by virtue ofexcellent adhesion of the polymer present in the organic/inorganiccomposite porous film according to the present invention. In this case,the adhesion can be controlled depending on the content of inorganicparticles and content and properties of the polymer. More particularly,as the polarity of the polymer increases and as the glass transitiontemperature (Tg) or melting point (Tm) of the polymer decreases, higheradhesion between the organic/inorganic composite porous film andelectrode can be obtained.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention. It is to be understood that the following examplesare illustrative only and the present invention is not limited thereto.

EXAMPLE 1-6

Preparation of Organic/Inorganic Composite Porous Film and Manufactureof Lithium Secondary Battery Using the Same

Example 1

1-1. Preparation of Organic/Inorganic Composite Porous Film(PVdF-CTFE/BaTiO₃)

PVdF-CTFE polymer (polyvinylidene fluoride-chlorotrifluoroethylenecopolymer) was added to acetone in the amount of about 5 wt % anddissolved therein at 50° C. for about 12 hours or more to form a polymersolution. To the polymer solution obtained as described above, BaTiO₃powder was added with the concentration of 20 wt % on the solid contentbasis. Then, the BaTiO₃ powder was pulverized into a size of about 300nm and dispersed for about 12 hours or more by using a ball mill methodto form slurry. Then, the slurry obtained as described above was coatedon a porous polyethylene terephthalate substrate (porosity: 80%) havinga thickness of about 20 μm by using a dip coating process to a coatinglayer thickness of about 2 μm. After measuring with a porosimeter, theactive layer impregnated into and coated on the porous polyethyleneterephthalate substrate had a pore size of 0.3 μm and a porosity of 55%.

1-2. Manufacture of Lithium Secondary Battery

(Manufacture of Cathode)

To N-methyl-2-pyrrolidone (NMP) as a solvent, 92 wt % of lithium cobaltcomposite oxide (LiCoO₂) as cathode active material, 4 wt % of carbonblack as conductive agent and 4 wt % of PVDF (polyvinylidene fluoride)as binder were added to form slurry for a cathode. The slurry was coatedon Al foil having a thickness of 20 μm as cathode collector and dried toform a cathode. Then, the cathode was subjected to roll press.

(Manufacture of Anode)

To N-methyl-2-pyrrolidone (NMP) as solvent, 96 wt % of carbon powder asanode active material, 3 wt % of PVDF (polyvinylidene fluoride) asbinder and 1 wt % of carbon black as conductive agent were added to formmixed slurry for an anode. The slurry was coated on Cu foil having athickness of 10 μm as anode collector and dried to form an anode. Then,the anode was subjected to roll press.

(Manufacture of Battery)

The cathode and anode obtained as described above were stacked with theorganic/inorganic composite porous film obtained as described in Example1-1 to form an assembly. Then, an electrolyte (ethylene carbonate(EC)/ethylemethyl carbonate (EMC)=1:2 (volume ratio) containing 1M oflithium hexafluorophosphate (LiPF₆)) was injected thereto to provide alithium secondary battery.

Example 2

Example 1 was repeated to provide a lithium secondary battery, exceptthat PMNPT (lead magnesium niobate-lead titanate) powder was usedinstead of BaTiO₃ powder to obtain an organic/inorganic composite porousfilm (PVdF-CTFE/PMNPT). After measuring with a porosimeter, the activelayer impregnated into and coated on the porous polyethyleneterephthalate substrate had a pore size of 0.4 μm and a porosity of 60%.

Example 3

Example 1 was repeated to provide a lithium secondary battery, exceptthat mixed powder of BaTiO₃ and Al₂O₃ (weight ratio=30:70) was usedinstead of BaTiO₃ powder to obtain an organic/inorganic composite porousfilm (PVdF-CTFE/BaTiO₃—Al₂O₃). After measuring with a porosimeter, theactive layer impregnated into and coated on the porous polyethyleneterephthalate substrate had a pore size of 0.2 μm and a porosity of 50%.

Example 4

Example 1 was repeated to provide a lithium secondary battery, exceptthat PVdF-CTFE was not used but about 2 wt % of carboxymethyl cellulose(CMC) polymer was added to water and dissolved therein at 60° C. forabout 12 hours or more to form a polymer solution, and the polymersolution was used to obtain an organic/inorganic composite porous film(CMC/BaTiO₃). After measuring with a porosimeter, the active layerimpregnated into and coated on the porous polyethylene terephthalatesubstrate had a pore size of 0.4 μm and a porosity of 58%.

Example 5

Example 1 was repeated to provide a lithium secondary battery, exceptthat neither PVdF-CTFE nor BaTiO₃ powder was used, and PVDF-HFP andlithium titanium phosphate (LiTi₂(PO₄)₃) powder were used to obtain anorganic/inorganic composite porous film (PVdF-HFP/LiTi₂ (PO₄)₃)comprising a porous polyethylene terephthalate substrate (porosity: 80%)having a thickness of about 20 μm coated with an active layer having athickness of about 2 μm. After measuring with a porosimeter, the activelayer impregnated into and coated on the porous polyethyleneterephthalate substrate had a pore size of 0.4 μm and a porosity of 58%.

Example 6

Example 1 was repeated to provide a lithium secondary battery, exceptthat neither PVdF-CTFE nor BaTiO₃ powder was used, and PVDF-HFP andmixed powder of BaTiO₃ and LiTi₂(PO₄)₃ (weight ratio=50:50) were used toobtain an organic/inorganic composite porous film(PVdF-HFP/LiTi₂(PO₄)₃—BaTiO₃). After measuring with a porosimeter, theactive layer impregnated into and coated on the porous polyethyleneterephthalate substrate had a pore size of 0.3 μm and a porosity of 53%.

COMPARATIVE EXAMPLES 1-3 Comparative Example 1

Example 1 was repeated to provide a lithium secondary battery, exceptthat a conventional poly propylene/polyethylene/polypropylene (PP/PE/PP)separator (see, FIG. 3) was used. The separator had a pore size of 0.01μm or less and a porosity of about 5%.

Comparative Example 2

Example 1 was repeated to provide a lithium secondary battery, exceptthat LiTi₂(PO₄)₃ and PVDF-HFP were used with a weight ratio of 10:90 toobtain an organic/inorganic composite porous film. After measuring witha porosimeter, the organic/inorganic composite porous film had a poresize of 0.01 μm or less and a porosity of about 5%.

Comparative Example 3

Example 1 was repeated to provide a lithium secondary battery, exceptthat PP/PE/PP separator was used as porous substrate, and BaTiO₃ andPVDF-HFP were used with a weight ratio of 10:90 to obtain anorganic/inorganic composite porous film. After measuring with aporosimeter, the organic/inorganic composite porous film had a pore sizeof 0.01 μm or less and a porosity of about 5%.

EXPERIMENTAL EXAMPLE 1 Surface Analysis of Organic/Inorganic CompositePorous Film

The following test was performed to analyze the surface of anorganic/inorganic composite porous film according to the presentinvention.

The sample used in this test was PVdF-CTFE/BaTiO₃ obtained according toExample 1. As control, a PP/PE/PP separator was used.

When analyzed by using a Scanning Electron Microscope (SEM), thePP/PE/PP separator according to Comparative Example 1 showed aconventional microporous structure (see, FIG. 3). On the contrary, theorganic/inorganic composite porous film according to the presentinvention showed a pore structure formed in the porous substrate itselfand pore structure formed by the surface of the porous substrate as wellas a part of the pores in the porous substrate, which are coated withinorganic particles (see, FIG. 2).

EXPERIMENTAL EXAMPLE 2 Evaluation of Heat Shrinkage of Organic/InorganicComposite Porous Film

The following experiment was performed to compare the organic/inorganiccomposite porous film with a conventional separator.

The organic/inorganic composite porous film (PVdF-CTFE/BaTiO₃) obtainedby using a heat resistant porous substrate according to Example 1 wasused as sample. The conventional PP/PE/PP separator and theorganic/inorganic composite porous film (PVdF-HFP/BaTiO₃) obtained byusing a conventional polyolefin-based separator according to ComparativeExample 3 were used as controls.

Each of the test samples were checked for its heat shrinkage afterstored at a high temperature of 150° C. for 1 hour. The test samplesprovided different results after the lapse of 1 hour at 150° C. ThePP/PE/PP separator as control was shrunk due to high temperature toleave only the outer shape thereof. Similarly, the film according toComparative Example 3 having a layer of inorganic particles formed onthe PP/PE/PP separator was shrunk significantly. This indicates thateven if heat resistant inorganic particles are used, a conventionalpolyolefine-based separator having poor thermal stability cannot provideimproved thermal safety by itself. On the contrary, theorganic/inorganic composite porous film according to the presentinvention showed good results with no heat shrinkage (see, FIG. 5)

As can be seen from the foregoing, the organic/inorganic compositeporous film according to the present invention has excellent thermalsafety.

EXPERIMENTAL EXAMPLE 3 Evaluation for Safety of Lithium SecondaryBattery

The following test was performed to evaluate the safety of each lithiumsecondary battery comprising the organic/inorganic composite porous filmaccording to the present invention.

Lithium secondary batteries according to Examples 1-6 were used assamples. As controls, used were the battery using a currently usedPP/PE/PP separator according to Comparative Example 1, the battery usingthe LiTi₂(PO₄)₃/PVdF-HFP film (weight ratio=10:90 on the wt % basis) asseparator according to Comparative Example 2, and the battery using thefilm comprising BaTiO₃/PVdF-HFP coating layer (weight ratio=10:90 on thewt % basis) formed on a currently used PP/PE/PP separator according toComparative Example 3.

3-1. Hot Box Test

Each battery was stored at high temperatures of 150° C. and 160° C. for1 hour and then checked. The results are shown in the following Table 1.

After storing at high temperatures, each of the batteries using acurrently used PP/PE/PP separator according to Comparative Examples 1and 3 caused explosion when stored at 160° C. for 1 hour. This indicatesthat polyolefin-based separators cause extreme heat shrinking, meltingand breakage when stored at high temperature, resulting in internalshort circuit between both electrodes (i.e., a cathode and anode) of abattery. On the contrary, lithium secondary batteries comprising theorganic/inorganic composite porous film according to the presentinvention showed such a safe state as to prevent firing and burning evenat a high temperature of 160° C. (see, Table 1).

Therefore, it can be seen that the lithium secondary battery comprisingan organic/inorganic composite porous film according to the presentinvention has excellent thermal safety. TABLE 1 Hot Box Test Conditions150° C./1 hr 160° C./1 hr Ex. 1 O O Ex. 2 O O Ex. 3 O O Ex. 4 O O Ex. 5O O Ex. 6 O O Comp. Ex. 1 O X Comp. Ex. 2 O O Comp. Ex. 3 O X

3-2. Overcharge Test

Each battery was charged under the conditions of 6V/1 A and 10V/1 A andthen checked. The results are shown in the following Table 2.

After checking the batteries comprising a currently used PP/PE/PPseparator according to Comparative Examples 1 and 3, they exploded (see,FIG. 6). This indicates that the polyolefin-based separator is shrunk byovercharge of the battery to cause short circuit between electrodes,resulting in degradation in safety of the battery. On the contrary, eachlithium secondary battery comprising an organic/inorganic compositeporous film according to the present invention showed excellent safetyunder overcharge conditions (see, Table 2 and FIG. 6). TABLE 2Overcharge Test Conditions 6 V/1 A 10 V/1 A Ex. 1 O O Ex. 2 O O Ex. 3 OO Ex. 4 O O Ex. 5 O O Ex. 6 O O Comp. Ex. 1 X X Comp. Ex. 2 O O Comp.Ex. 3 X X

EXPERIMENTAL EXAMPLE 4 Evaluation for Quality of Lithium SecondaryBattery

The following tests were performed in order to evaluate high-ratedischarge characteristics and cycle characteristics of each lithiumsecondary battery comprising an organic/inorganic composite porous filmaccording to the present invention.

4-1. Evaluation for C-Rate Characteristics

Lithium secondary batteries according to Examples 1-6 were used assamples. As controls, used were the battery using a currently usedPP/PE/PP separator according to Comparative Example 1, the battery usingthe LiTi₂(PO₄)₃/PVdF-HFP film (weight ratio=10:90 on the wt % basis) asseparator according to Comparative Example 2, and the battery using thefilm comprising BaTiO₃/PVdF-HFP coating layer (weight ratio=10:90 on thewt % basis) formed on a currently used PP/PE/PP separator according toComparative Example 3.

Each battery having a capacity of 760 mAh was subjected to cycling at adischarge rate of 0.5 C, 1 C and 2 C. The following Table 3 shows thedischarge capacity of each battery, the capacity being expressed on thebasis of C-rate characteristics.

After performing the test, the batteries according to ComparativeExamples 2 and 3, each using, as separator, an organic/inorganiccomposite porous film that includes a mixture containing inorganicparticles with a high dielectric constant or inorganic particles withlithium ion conductivity and a binder polymer in a ratio of 10:90 (onthe wt % basis), showed a significant drop in capacity depending ondischarge rates, as compared to the batteries using, as separators, theorganic/inorganic composite porous film obtained from the above Examplesaccording to the present invention and a conventional polyolefin-basedseparator (see, Table 3). This indicates that such relatively low amountof inorganic particles compared to the polymer may decrease the poresize and porosity in the pore structure formed by interstitial volumeamong the inorganic particles, resulting in degradation in the qualityof a battery.

On the contrary, lithium secondary batteries comprising theorganic/inorganic composite porous film according to the presentinvention showed C-rate characteristics comparable to those of thebattery using a conventional polyolefin-based separator under adischarge rate of up to 2 C (see, Table 3 and FIG. 7). TABLE 3 DischargeRate (mAh) 0.5 C 1 C 2 C Ex. 1 756 745 692 Ex. 2 757 747 694 Ex. 3 758746 693 Ex. 4 755 742 691 Ex. 5 756 745 792 Ex. 6 757 747 791 Comp. Ex.1 752 741 690 Comp. Ex. 2 630 582 470 Comp. Ex. 3 612 551 434

4-2. Evaluation for Cycle Characteristics

The lithium secondary battery using the organic/inorganic compositeporous film (PVdF-CTFE/BaTiO₃) according to Example 1 and the lithiumsecondary battery using a currently used PP/PE/PP separator according toComparative Example 1 were used. Each battery was charged at atemperature of 23° C. with an electric current of 0.5 C under a voltageof 4.2-3V. Then, initial capacity of each battery was measured and eachbattery was subjected to 300 charge/discharge cycles.

After performing the test, the lithium secondary battery using anorganic/inorganic composite porous film according to the presentinvention as separator showed an efficiency of 80% or more even after300 cycles. In other words, the lithium secondary battery according tothe present invention showed cycle characteristics comparable to thoseof the battery using a conventional polyolefin-based separator (see,FIG. 8). Therefore, it can be seen that the electrochemical devicecomprising the organic/inorganic composite porous film according to thepresent invention shows long service life.

INDUSTRIAL APPLICABILITY

As can be seen from the foregoing, the organic/inorganic compositeporous film according to the present invention can solve the problem ofpoor thermal safety occurring in a conventional polyolefin-basedseparator by using a heat resistant porous substrate. Additionally, theorganic/inorganic composite porous film according to the presentinvention has pore structures formed in the porous substrate itself aswell as in the active layer formed on the substrate by using inorganicparticles and a binder polymer, thereby increasing the space to befilled with an electrolyte, resulting in improvement in a degree ofswelling with electrolyte and lithium ion conductivity. Therefore, theorganic/inorganic composite porous film according to the presentinvention contributes to improve the thermal safety and quality of alithium secondary battery using the same as separator.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment and the drawings. On the contrary, it is intendedto cover various modifications and variations within the spirit andscope of the appended claims.

1. An organic/inorganic composite porous film, which comprises: (a) aporous substrate having pores; and (b) an active layer formed by coatinga surface of the substrate or a part of the pores in the substrate witha mixture of inorganic particles and a binder polymer, wherein theinorganic particles in the active layer are interconnected amongthemselves and are fixed by the binder polymer, and interstitial volumesamong the inorganic particles form a pore structure.
 2. The filmaccording to claim 1, wherein the inorganic particles are at least oneselected from the group consisting of: (a) inorganic particles having adielectric constant of 5 or more; and (b) inorganic particles havinglithium ion conductivity.
 3. The film according to claim 2, wherein theinorganic particles having a dielectric constant of 5 or more areBaTiO₃, Pb(Zr,Ti)O₃ (PZT), Pb_(1-x)La_(x)Zr_(1-y)Ti_(y)O₃ (PLZT),Pb(Mg₃Nb_(2/3))O₃—PbTiO₃ (PMN-PT), hafnia (HfO₂), SrTiO₃, SnO₂, CeO₂,MgO, NiO, CaO, ZnO, ZrO₂, Y₂O₃, Al₂O₃ or TiO₂.
 4. The film according toclaim 2, wherein the inorganic particles having lithium ion conductivityare at least one selected from the group consisting of: lithim phosphate(Li₃PO₄), lithium titanium phosphate (Li_(x)Ti_(y)(PO₄)₃, 0<x<2, 0<y<3),lithium aluminium titanium phosphate (Li_(x)Al_(y)Ti_(z)(PO₄)₃, 0<x<2,0<y<1, 0<z<3), (LiAlTiP)_(x)O_(y) type glass (0<x<4, 0<y<13), lithiumlanthanum titanate (Li_(x)La_(y)TiO₃, 0<x<2, 0<y<3), lithium germaniumthiophosphate (Li_(x)Ge_(y)P_(z)S_(w), 0<x<4, 0<y<1, 0<z<1, 0<w<5),lithium nitrides (Li_(x)N_(y), 0<x<4, 0<y<2), SiS₂ type glass(Li_(x)Si_(y)S_(z), 0<x<3, 0<y<2, 0<z<4) and P₂S₅ type glass(Li_(x)P_(y)S_(z), 0<x<3, 0<y<3, 0<z<7).
 5. The film according to claim1, wherein the inorganic particles have a size of between 0.001 μm and10 μm.
 6. The film according to claim 1, wherein the inorganic particlesare present in the mixture of inorganic particles with the binderpolymer in an amount of 50-99 wt % based on 100 wt % of the mixture. 7.The film according to claim 1, wherein the binder polymer has a glasstransition temperature (Tg) of between −200° C. and 200° C.
 8. The filmaccording to claim 1, wherein the binder polymer has a solubilityparameter of between 15 and 45 MPa^(1/2).
 9. The film according to claim1, wherein the binder polymer is at least one selected from the groupconsisting of polyvinylidene fluoride-co-hexafluoropropylene,polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate,polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate,polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate,cellulose acetate butyrate, cellulose acetate propionate,cyanoethylpullulan, cyanoethyl polyvinylalcohol, cyanoethylcellulose,cyanoethylsucrose, pullulan, carboxymetyl cellulose,acrylonitrile-styrene-butadiene copolymer and polyimide.
 10. The filmaccording to claim 1, wherein the porous substrate having pores shows amelting point of 200° C. or higher.
 11. The film according to claim 10,wherein the porous substrate having a melting point of 200° C. or higheris at least one selected from the group consisting of polyethyleneterephthalate, polybutylene terephthalate, polyester, polyacetal,polyamide, polycarbonate, polyimide, polyetherether ketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfidro and polyethylenenaphthalene.
 12. The film according to claim 1, wherein the poroussubstrate has a pore size of between 0.01 and 50 μm and theorganic/inorganic composite porous film has a pore size of between 0.01and 10 μm.
 13. The film according to claim 1, which has a porosity ofbetween 5% and 95%.
 14. The film according to claim 1, which has athickness of between 1 and 100 μm.
 15. An electrochemical devicecomprising: (a) a cathode; (b) an anode; (c) an organic/inorganiccomposite porous film as defined in claim 1, which is interposed betweenthe cathode and anode; and (d) an electrolyte.
 16. The electrochemicaldevice according to claim 15, which is a lithium secondary battery. 17.The electrochemical device according to claim 15, which furthercomprises a microporous separator.
 18. The electrochemical deviceaccording to claim 17, wherein the microporous separator is apolyolefin-based separator.
 19. A method for manufacturing anorganic/inorganic composite porous film as defined in claim 1, whichcomprises the steps of: (a) dissolving a binder polymer into a solventto form a polymer solution; (b) adding inorganic particles to thepolymer solution obtained from step (a) and mixing them; and (c) coatingthe mixture of inorganic particles with binder polymer obtained fromstep (b) on the surface of a substrate having pores or on a part of thepores in the substrate, followed by drying.